The electromagnetic radio spectrum is a natural resource, the use of which by transmitters and receivers is licensed by governments. In many bands, spectrum access is a more significant problem than physical scarcity of spectrum, in large part due to legacy command-and-control regulation that limits the ability of potential spectrum users to obtain such access. Indeed, if portions of the radio spectrum were scanned, including in the revenue-rich urban areas, one would find that some frequency bands in the spectrum are largely unoccupied most of the time; some other frequency bands are only partially occupied; and the remaining frequency bands are heavily used.
The underutilization of the electromagnetic spectrum has led to the view that spectrum holes within the electromagnetic spectrum exist. As used herein, a spectrum hole exists when a band of frequencies assigned to a primary user is not being utilized by that user, at a particular time and specific geographic location. By making it possible for a secondary user to access the band of frequencies within a spectrum hole, utilization of the electromagnetic spectrum may be improved. A cognitive radio, inclusive of software-defined radio, has been proposed as a means to promote the efficient use of the electromagnetic spectrum by exploiting the existence of spectrum holes.